1. Field of the Invention
This invention relates to a method of manufacturing semiconductor devices in which there is a strong electrical and mechanical bond between the semiconductor element and the supporting electrode.
2. Description of the Prior Art
In order to achieve superior ohmic contact between a semiconductor element and its electrode, it is common practice in the manufacture of the semiconductor to deposit a metal layer of aluminum, chromium or nickel on one of the main surfaces of the semiconductor where the electrode is to be provided. On the other hand, the impurity concentration in the surface of the semiconductor where the metal layer is to be provided is increased to form therein an N-type or P-type high concentration surface layer.
The bond or adhesion of the metal layer to the N-type high concentration surface layer is very weak in a certain direction, and it is known that the metal layer tends to be peeled off from the semiconductor element by the mechanical stresses which may be applied thereto in the processes of manufacture thereof.
The inventor has made a study of the causes of the peeling off of the metal layer and has discovered the fact mentioned below.
The surface of the N-type high concentration layer is oversupplied with electrons which adsorb ions of impurities prior to the depositing of the metal layer, resulting in a lower adhesive or bonding power. When phosphorus is used as a doping material for the N-type high concentration layer, the surface of the semiconductor element is transformed into P.sub.2 O.sub.5 by heat applied at the time of doping. This P.sub.2 O.sub.5 absorbs much water, causing the unsatisfactory adhesive or bonding power of the metal layer.
Also, it has been ascertained that the peeling off of the metal layer occurs especially when required PN junctions are formed in a semiconductor wafer, followed by the pelletization of the wafer to obtain a plurality of semiconductor elements.
In the manufacture of semiconductor elements of 0.5 to 1.5 mm by 0.5 to 1.5 mm, for example, the masking tape method is now in use to attain an improved operating efficiency of scribe-cut and sorting work. In this masking tape method, special stress is applied to the metal layer, often causing it to be peeled off.